In recent years, organic electronic devices such as organic electroluminescent devices (“organic EL device(s)” hereinafter) and organic solar cells have attracted attention. For any of these organic electronic devices, a transparent electrode is an essential component.
A conventional transparent electrode having been widely used is an ITO transparent electrode manufactured by forming an indium-tin composite oxide (abbreviated to “ITO” hereinafter) film by vacuum deposition or sputtering on a transparent substrate because of its advantages in properties such as conductivity and transparency.
However, the transparent electrode manufactured by vacuum deposition or sputtering is low in productivity, which brings a financial problem of high manufacturing costs. Further, in recent years, there has been a strong demand for organic electronic devices having a larger area. Such organic electronic devices require a large vacuum facility for vacuum deposition, sputtering or the like, which is a great financial burden. In addition, resistance of an ITO transparent electrode has been insufficient.
For the area increase and also for organic electronic devices which are demanded to have a low resistance, there has been developed a transparent electrode composed of: a thin metal wire formed in a pattern; and a transparent conductive layer composed of a conductive polymer and the like stacked on the thin metal wire, and having both current uniformity in surface and high conductivity (refer to Patent Documents 1 and 2, for example).
However, with the method described in Patent Document 1, the coated film of ITO dispersoids itself used for forming the transparent conductive layer is not smooth, and hence there is a problem of current leakage in the case where the transparent electrode is used in an organic electronic device. Meanwhile, with the method described in Patent Document 2, smoothness of the conductive polymer itself has no problem, but it is necessary to smoothen asperity of the thin metal wire with the transparent conductive layer composed of the conductive polymer and the like, and therefore it is essential to thicken the conductive polymer. A conductive polymer, however, shows absorption in the visible light region. Therefore, if the conductive polymer is thickened, the transparency of the transparent electrode significantly decreases.
As a method to achieve both the conductivity and the transparency, which is a problem as described above, there is disclosed a method of using a hydroxyalkyl group-containing acrylic polymer such as poly(hydroxyethyl acrylate) for a transparent conductive layer (refer to Patent Document 3, for example).
However, the application liquid containing the conductive polymer and the hydroxyalkyl group-containing acrylic polymer, which is described in Patent Document 3, increases its viscosity with the lapse of time, and hence there is a problem in stability of the application liquid over time, so-called pot life. In particular, in the case where a transparent conductive layer is formed by inkjet printing, if an application liquid having such viscosity variability is used and ejected continuously, nozzle clogging occurs at a part of an inkjet recording head(s) with viscosity increase when the ejection is carried out several ten times. Thus, ejection stability cannot be maintained.
In addition, although it depends on a solvent to select, there are problems that: a gel insoluble matter is generated in the application liquid; and at a step of drying the application liquid, the coated film is contracted and therefore cannot keep a desired shape which is a shape right after the application, namely, a designed shape. If a transparent electrode manufactured by using the application liquid in which a gel insoluble matter is generated is used in an organic electronic device, current leakage occurs in the organic electronic device, and a rectification ratio thereof substantially decreases. In addition, the application liquid is wet-spread at the drying step, and therefore pattern accuracy decreases, and also current leakage in wiring may be caused. Meanwhile, if the formed coated film is contracted by being dried, the area of the transparent conductive layer decreases, so that the edges of the thin metal wire are not covered therewith and are exposed therefrom, which leads to decrease in the rectification ratio of an organic electronic device.
As a method for giving a conductive polymer onto a substrate having asperity by inkjet printing, there is also disclosed a method of using a solvent such as an ethylene glycol or an ethylene glycol ether (refer to Patent Document 4, for example).
However, even with the method described in Patent Document 4, in the existing circumstances, not all of the above problems have been solved yet.